In general, an I-type prestressed concrete girder bridge having an I-shaped section is one of bridges that are inexpensive and widely used in the world. However, for the I-type girder bridge, when the length of a girder exceeds 40-50 m, the length and weight of the girder increase so that all processes such as manufacturing, carrying and installing of the girder become very difficult. Thus, for a concrete box type girder, a bridge is constructed in a method of connecting a plurality of splice members that are as short as 2-3 m.
In the conventional I-type girder bridge, since steel wires are mostly installed at an end portion of a girder, the steel wires cannot be installed according to the moment distribution property which increases in the middle portion of the girder so that the amount of steel wires and the size of the section of the girder increase as a whole. Also, since the size of the end portion of the girder needs to be made large, the weight of the girder increases and the manufacture of a mold becomes difficult.
Conventionally, to efficiently construct a long span I-type girder bridge, a girder is manufactured by dividing into a plurality of splices and the spliced girders are moved to a construction site and combined together there. For the concrete box girder bridge, a construction method using 15 or more splices per span is generally and widely used. However, for the I-type girder, it is very rare to use the spliced girder and such a method is used only for a small bridge or at a construction site such as a mountain area where the construction is difficult. In particular, in the spliced girder bridge, when a load is applied, destruction starts in a spliced portion so that the load carrying capacity of the spliced girder bridge is lowered by 20-30% compared to an integral type girder bridge and it can be said that the spliced girder bridge has an inefficient structural property.
Furthermore, in a conventional incrementally prestressed girder bridge, the installation position of an anchoring device of steel wires for prestressing is limited to the side surface of the end portion of the girder. Thus, since the installation position of the anchoring device is not free, the dynamically efficient arrangement of steel wires is difficult in view of the whole girder.
Also, since the conventional girder bridge has a structure in which a body portion is closed, not only the weight increases but also the bridge shows weakness to a load such as wind or water acting in a direction perpendicular to the girder. Also, the closed body portion of the girder gives a feeling of being locked in.